In wavelength conversion devices using nonlinear optical effects, light that is to be converted is input into a nonlinear optical crystal that exhibits nonlinear optical effects, and an important point is that the converted light that arises due to the nonlinear optical effects should be efficiently and stably outputted.
As one example, a wavelength conversion device that uses second harmonic generation (SHG) shall be explained. Therefore, in the following explanation, the input light may at times be called fundamental wave light, and the output light may at times be called converted light or SH light. Of course, nonlinear optical effects that are used for performing wavelength conversion are not restricted to SHG, there being wavelength conversion using other second-order nonlinear optical effects, similarly with SHG, such as differential frequency generation (DFG) or sum frequency generation (SFG), and additionally, wavelength conversion that uses higher order nonlinear optical effects of third-order or higher. The fundamental constitution that generates converted light efficiently in the wavelength conversion device using SHG explained below, can similarly be used, of course, in wavelength conversion devices that use second-order nonlinear optical effects, such as DFG and SFG described above, and not only these but also wavelength conversion devices that use third or higher order nonlinear optical effects.
In the wavelength conversion device using SHG described above, it is necessary to irradiate fundamental wave light at an angle that satisfies the phase matching conditions relative to the crystal axis of crystals, such as LiNbO3 (lithium niobate) and BBO (β-BaB2O4: beta barium borate) that produce nonlinear optical effects (here, SHG). That is, in the above-mentioned wavelength conversion device, it is necessary that the direction of propagation of fundamental wave light relative to the crystal axis of the nonlinear optical crystal is consistent with an angle satisfying the phase matching conditions. Hereinafter, when the fundamental wave light enters at an angle (phase matching angle) that satisfies the phase matching conditions relative to the crystal axis of the nonlinear optical crystal, we shall, at times, simply say “enters while satisfying the phase matching angle.” If the light can be made to enter while satisfying the phase matching angle, the SHG efficiency, that is, the wavelength conversion efficiency, can be maximized.
In order to make the light enter a nonlinear optical crystal while satisfying the phase matching angle, an optical path adjusting portion, constituted by appropriately placing reflecting mirrors and/or prisms and/or lenses and the like between the nonlinear optical crystal and the light source (laser light source) that generates the fundamental wave light, is provided. The direction of propagation of the fundamental wave light is adjusted using this optical path adjusting portion. Hereinafter, the light source that generates the fundamental wave light will be assumed to be a laser light source, and the placement configuration of the nonlinear optical crystal, reflecting mirrors, prisms, or lenses and the like shall be called the optical system of the wavelength conversion device.
The phase matching conditions are prescribed by the shape of the index ellipsoid of the nonlinear optical crystal relative to the fundamental wave light, and the shape of this index ellipsoid depends upon the temperature of the nonlinear optical crystal. The temperature of the nonlinear optical crystal constantly fluctuates during the operation of the wavelength conversion device, due to the effects of changes such as in ambient temperature. Therefore, in order to maintain the SHG conversion efficiency of the wavelength conversion device constantly at a maximum, it is necessary to constantly adjust the angle of incidence of the fundamental wave light relative to the crystal axis of the nonlinear optical crystal.
Additionally, in the manufacturing process of the wavelength conversion device, during the task of completing the optical system thereof by adjusting it so as to be optimal, in order to maximize the wavelength conversion efficiency, it is necessary to adjust the optical path of the fundamental wave light (direction of propagation of fundamental wave light) by adjusting the position, orientation, and the like of reflecting mirrors and the like that comprise the optical system. In addition, a wavelength conversion device, even after it has been completed after having been adjusted so that the wavelength conversion efficiency thereof is maximized, when the wavelength conversion device is run and its operation is then halted, when running it again afterwards, it will not be the case that the wavelength conversion efficiency of the device is reproduced in a maximum state. That is, the optical system of the wavelength conversion device must be adjusted so that the wavelength conversion efficiency is maximized, not only during the assembly process, but also whenever it is started to operate, and not only this, but it is necessary constantly to adjust the optical system during operation itself.
In the above-mentioned optical path adjusting portion, in order to adjust the optical path of the fundamental wave light, that is, in order to adjust the direction of propagation of fundamental wave light that enters the nonlinear optical crystal, an optical fine motion device equipped with a reflecting mirror or the like, such as a gimbal fine motor device is used to adjust the reflecting surface of the reflecting mirror and the angle of incidence of the fundamental wave light into this reflecting mirror. This fine motion adjusting device can make the reflecting surface of the reflecting mirror rotate, as well as undergo parallel translational motion, relative to the fundamental wave light that is incident upon the reflecting surface of the reflecting mirror. Parallel translational motion requires control of two axes in order to move in perpendicular x and y directions, and additionally, rotation also requires control of two axes in order to perform rotation around both the x axis and the y axis. That is, control of a total of four axes is necessary.
The above-mentioned task is generally called “alignment.” Additionally, the optical path adjusting portion described above is constituted by combining a plurality of reflecting mirrors, or a plurality of prisms or the like. Therefore, alignment must be performed relative to a plurality of optical fine motion devices. The task of manipulating a plurality of optical fine motion devices, and determining the optimal optical path relative to the fundamental wave light is very difficult, and generally, the manual labor of experienced workers is relied upon. The reason that adjusting the optical path of a fundamental wave light by manipulating a plurality of optical fine motion devices is a highly difficult art is because during the act of adjusting a plurality of an optical fine motion devices, it is not the case that each optical fine motion device respectively determines the optical path of the fundamental wave light independently, but rather they are mutually dependent, so they do not respectively contribute independently to the determination of the optical path.
The alignment task mentioned above is, as already mentioned, necessary not only at the stage of manufacturing the wavelength conversion device, but also at each time the wavelength conversion device is utilized. This is because, due to changes of the ambient temperature in the place where the wavelength conversion device is installed and the like, tiny changes occur in relative positions and the like within the optical system of the wavelength conversion device. Due to these tiny changes, the angle of incidence of the fundamental wave light into the nonlinear optical crystal fluctuates, and becomes offset from the phase matching conditions, thereby lowering the wavelength conversion efficiency. Therefore, alignment is constantly necessary. Of course, in order to operate the wavelength conversion device under conditions that satisfy the phase matching requirements, due to the same reasons as those described above (changes in the ambient temperature of the wavelength conversion device and the like), alignment is necessary.
Therefore, during the manufacture of the wavelength conversion device, in order to adjust the optical path efficiently, and additionally, in order to realize the maximum wavelength conversion efficiency during operation or re-operation of the wavelength conversion device, the realization of a wavelength conversion device that can realize automatic control, by providing an optical path adjusting portion, is desired.
The cases where, in order to adjust the position and direction of propagation of a laser beam, the adjustment of a plurality of adjustment means is carried out in parallel, are not restricted merely to the wavelength conversion device described above. Cases where the adjustment of a plurality of adjustment means must be carried out in parallel in order to adjust an optical system include, for example, optical systems that are incorporated into exposure devices (also sometimes called steppers) that are used during semiconductor manufacture, or optical systems including line image sensors incorporated in manuscript reading devices, and the like.
Regarding methods or devices for automatically adjusting optical systems incorporated into exposure devices used during semiconductor manufacture, methods and devices have been disclosed that irradiates light on an alignment mark formed on a semiconductor wafer, and automatically detects the position of the alignment mark based upon a signal obtained by photoelectric conversion of the reflected light from the alignment mark (see, e.g., Patent Citation 1 and 2).
Additionally, regarding optical systems including line image sensors incorporated into manuscript reading devices, a device has been disclosed that automatically performs, in a device which reads in an image of a manuscript by image formation on a line image sensor using an image formation lens, the alignment of the focal point location, and the side registration, skew, or the slant of the manuscript (see, e.g., Patent Citation 3).
In each of these inventions, a control method based upon fuzzy inference has been used for the automation of the alignment, and thereby, simple and highly accurate alignment has been realized.    [Patent Citation 1] Japanese Patent No. 2517637    [Patent Citation 2] Japanese Patent Application Publication No. H09-232232    [Patent Citation 3] Japanese Patent No. 3077303